1 Technical Field of the Invention
The present invention relates generally to a fuel cell system equipped with a fuel cell working to produce an electric energy arising from chemical reaction of hydrogen with oxygen which may be employed in mobile objects such as automotive vehicles, ships or portable generator, and more particularly to an improved structure of such a fuel cell system designed to ensure desired startability.
2 Background Art
In order to improve a utility factor of fuel and power generation efficiency of fuel cells, off-gas recirculation systems are known which work to suck the off-gas emitted from hydrogen electrodes of the fuel cells using a pump and mix it with fuel supplied to the fuel cells. The pump used to recirculate the off-gas is usually implemented by an ejector pump because it can be actuated by fluid energy of the fuel and is of great advantage to power saving. For instance, Japanese Patent No. 2001-266922 teaches such a system.
Fluid-driven pumps such as ejector pumps, however, have the problem in that when the amount of fuel supplied to the fuel cells is small, that is, when fluid energy required for recirculating the off-gas is low, it results in a lack of a recirculated amount of the off-gas. Particularly, when the fuel cells are at rest during idling stops of the engine, the supply of fuel to the fuel cells is stopped. It is, thus, impossible to recirculate the off-gas.
Usually, during suspension of power generation in the fuel cells, transmission of air through electrolyte films of the fuel cells causes impurities such as nitrogen or water produced by cell reaction are accumulated within an off-gas recirculation path of the system. Additionally, when the amount of the off-gas is insufficient, it is difficult to remove the impurities from the fuel cells, thus promoting accumulation of the impurities within the off-gas recirculation path. The accumulated impurities disturb reaction of the electrodes of the fuel cells, thus resulting in decreases in startability and power generation capability of the fuel cells.